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Excited-state intramolecular proton transfer in 2-(3'-hydroxy-2'-pyridyl)benzoxazole. Evidence of coupled proton and charge transfer in the excited state of some o-hydroxyarylbenzazoles.
J Phys Chem A. 2007 Mar 15; 111(10):1814-26.JP

Abstract

The influence of solvent, temperature, and viscosity on the phototautomerization processes of a series of o-hydroxyarylbenzazoles was studied by means of ultraviolet-visible (UV-vis) absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. The compounds studied were 2-(2'-hydroxyphenyl)benzimidazole (HBI), 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzothiazole (HBT), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (HPyBI), and the new derivative 2-(3'-hydroxy-2'-pyridyl)benzoxazole (HPyBO), this one studied in neutral and acid media. All of these compounds undergo an excited-state intramolecular proton transfer (ESIPT) from the hydroxyl group to the benzazole N3 to yield an excited tautomer in syn conformation. A temperature- and viscosity-dependent radiationless deactivation of the tautomer has been detected for all compounds except HBI and HPyBI. We show that this radiationless decay also takes place for 2-(3-methyl-1,3-benzothiazol-3-ium-2-yl)benzenolate (NMeOBT), the N-methylated analog of the tautomer, whose ground-state structure has anti conformation. In ethanol, the radiationless decay shows intrinsic activation energy for HPyBO and HBO; however, it is barrierless for HBT and NMeOBT and controlled instead by the solvent dynamics. The relative efficiency of the radiationless decay in the series of molecules studied supports the hypothesis that this transition is connected with a charge-transfer process taking place in the tautomer, its efficiency being related to the strength of the electron donor (dissociated phenol or pyridinol moiety) and electron acceptor (protonated benzazole). We propose that the charge transfer is associated with a large-amplitude conformational change of the tautomer, the process leading to a nonfluorescent charge-transfer intermediate. The previous ESIPT step generates the structure with the suitable redox pair to undergo the charge-transfer process; therefore, an excited-state intramolecular coupled proton and charge transfer takes place for these compounds.

Authors+Show Affiliations

Departamento de Química Física, Facultade de Química, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain.No affiliation info availableNo affiliation info availableNo affiliation info available

Pub Type(s)

Journal Article

Language

eng

PubMed ID

17309239

Citation

Vázquez, Sonia Ríos, et al. "Excited-state Intramolecular Proton Transfer in 2-(3'-hydroxy-2'-pyridyl)benzoxazole. Evidence of Coupled Proton and Charge Transfer in the Excited State of some O-hydroxyarylbenzazoles." The Journal of Physical Chemistry. A, vol. 111, no. 10, 2007, pp. 1814-26.
Vázquez SR, Rodríguez MC, Mosquera M, et al. Excited-state intramolecular proton transfer in 2-(3'-hydroxy-2'-pyridyl)benzoxazole. Evidence of coupled proton and charge transfer in the excited state of some o-hydroxyarylbenzazoles. J Phys Chem A. 2007;111(10):1814-26.
Vázquez, S. R., Rodríguez, M. C., Mosquera, M., & Rodríguez-Prieto, F. (2007). Excited-state intramolecular proton transfer in 2-(3'-hydroxy-2'-pyridyl)benzoxazole. Evidence of coupled proton and charge transfer in the excited state of some o-hydroxyarylbenzazoles. The Journal of Physical Chemistry. A, 111(10), 1814-26.
Vázquez SR, et al. Excited-state Intramolecular Proton Transfer in 2-(3'-hydroxy-2'-pyridyl)benzoxazole. Evidence of Coupled Proton and Charge Transfer in the Excited State of some O-hydroxyarylbenzazoles. J Phys Chem A. 2007 Mar 15;111(10):1814-26. PubMed PMID: 17309239.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Excited-state intramolecular proton transfer in 2-(3'-hydroxy-2'-pyridyl)benzoxazole. Evidence of coupled proton and charge transfer in the excited state of some o-hydroxyarylbenzazoles. AU - Vázquez,Sonia Ríos, AU - Rodríguez,M Carmen Ríos, AU - Mosquera,Manuel, AU - Rodríguez-Prieto,Flor, Y1 - 2007/02/20/ PY - 2007/2/21/pubmed PY - 2007/2/21/medline PY - 2007/2/21/entrez SP - 1814 EP - 26 JF - The journal of physical chemistry. A JO - J Phys Chem A VL - 111 IS - 10 N2 - The influence of solvent, temperature, and viscosity on the phototautomerization processes of a series of o-hydroxyarylbenzazoles was studied by means of ultraviolet-visible (UV-vis) absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. The compounds studied were 2-(2'-hydroxyphenyl)benzimidazole (HBI), 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzothiazole (HBT), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (HPyBI), and the new derivative 2-(3'-hydroxy-2'-pyridyl)benzoxazole (HPyBO), this one studied in neutral and acid media. All of these compounds undergo an excited-state intramolecular proton transfer (ESIPT) from the hydroxyl group to the benzazole N3 to yield an excited tautomer in syn conformation. A temperature- and viscosity-dependent radiationless deactivation of the tautomer has been detected for all compounds except HBI and HPyBI. We show that this radiationless decay also takes place for 2-(3-methyl-1,3-benzothiazol-3-ium-2-yl)benzenolate (NMeOBT), the N-methylated analog of the tautomer, whose ground-state structure has anti conformation. In ethanol, the radiationless decay shows intrinsic activation energy for HPyBO and HBO; however, it is barrierless for HBT and NMeOBT and controlled instead by the solvent dynamics. The relative efficiency of the radiationless decay in the series of molecules studied supports the hypothesis that this transition is connected with a charge-transfer process taking place in the tautomer, its efficiency being related to the strength of the electron donor (dissociated phenol or pyridinol moiety) and electron acceptor (protonated benzazole). We propose that the charge transfer is associated with a large-amplitude conformational change of the tautomer, the process leading to a nonfluorescent charge-transfer intermediate. The previous ESIPT step generates the structure with the suitable redox pair to undergo the charge-transfer process; therefore, an excited-state intramolecular coupled proton and charge transfer takes place for these compounds. SN - 1089-5639 UR - https://www.unboundmedicine.com/medline/citation/17309239/Excited_state_intramolecular_proton_transfer_in_2__3'_hydroxy_2'_pyridyl_benzoxazole__Evidence_of_coupled_proton_and_charge_transfer_in_the_excited_state_of_some_o_hydroxyarylbenzazoles_ L2 - https://doi.org/10.1021/jp0653813 DB - PRIME DP - Unbound Medicine ER -
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